It is well known to use accelerometers for navigation of aircraft, rockets, and guided weaponry such as missiles. Also, it is known to use silicon in accelerometers and to use a dual range accelerometer.
U.S. Pat. No. 4,346,597 teaches an accelerometer including a mass supported on a cantilevered beam with stops on either side. For the case of low acceleration, the beam acts as a cantilever while at high acceleration, the beam is caused to flex as a beam supported at both ends. This invention is capable of detecting accelerations in two different ranges.
Other U.S. patents teach accelerometers of dual range acceleration detection. In U.S. Pat. No. 4,611,491, a system is disclosed which provides a low-frequency inertial quality force feedback accelerometer as well as a high-frequency accelerometer. The high-frequency accelerometer is typically a piezo electric accelerometer. U.S. Pat. No. 4,712,427 discloses a dual sensor, frequency output accelerometer. The reference teaches an accelerometer having a first sensor which produces an output signal S.sub.1 with a frequency F.sub.1 and a second sensor which provides an output signal S.sub.2 with a frequency F.sub.2. The sensors are arranged so that a given acceleration causes the frequency of the one output signal to increase while the frequency of the other output signal decreases.
Several U.S. patent disclose silicon accelerometer. U.S. Pat. No. 4,660,418 discloses a flexible hinge device comprising a laminar silicon body having a groove portion etched away so as to define a thin flexible strip joining two rigid end pieces. A piezo resistive accelerometer is disclosed U.S. Pat. No. 4,430,895. The device is formed by etching a silicon wafer to form individual acceleration sensing elements. In U.S. Pat. No. 4,553,436, a silicon accelerometer is disclosed which employs the piezo resistive effect to measure the flexure of semiconductor beams supporting a semiconductor mass.
The accelerometers disclosed in U.S. Pat. Nos. 4,131,020 and 4,094,199 disclose a capacitive pick-off Also of note in these two patents is FIG. 14 which teaches a circuit having a high pass filter built into it. The present invention has a high pass filter as an equivalent circuit for the disclosed accelerometer. The high pass filter is built into FIG. 14 as capacitor 352 and resistor 354, but since this drawing also has a low pass filter built in, it operates as a band pass filter.
It is an object of the present invention to successfully measure acceleration from steady state to 500 Hertz and beyond. Known piezo resistive methods involve going through a 180.degree. phase shift between 200 and 500 Hertz which is undesirable. A solution to this problem is to use the piezo resistive pickoff for low frequency response, less than 100 Hertz, and to use a secondary pickoff for response above 100 Hertz.
The integrated silicon accelerometer, as disclosed in U.S. Pat. No. 4,488,445, is a precision accelerometer for navigation of aircraft, rockets, or missiles. The teachings of commonly assigned U.S. Pat. Nos. 4,488,445 and 4,498,342 are hereby incorporated by reference.